The mutagenic and carcinogenic activities of previously untested electrophilic agents were investigated. Salmonella TA100 was used in standard plate tests to determine the mutagenic activities of the alkyl halides. The ability of alkyl halides to produce lung adenomas in strain A mice was also studied. In general, branching and ease of dissociation increased both the mutagenic and carcinogenic activities of the alkyl halides. Nucleophilic substitution tended to increase the toxicity, the mutagenicity and the tumorigenic activity of the alkyl halides. The effect of metal carcinogens on DNA synthesis was investigated.